III-N single crystals are of great technical importance. A multitude of semiconductor devices and optoelectronic devices such as power components, high-frequency components, light-emitting diodes and lasers are based on these materials. Epitaxial crystal growth on a starting substrate is frequently carried out when producing such devices, or a template is initially formed on a starting substrate, onto which III-N layers or respectively III-N boules can be subsequently deposited by further epitaxial growth. III-N substrates or in particular foreign substrates can be used as starting substrates. When using foreign substrates, stresses and cracks within a III-N layer can occur during the growth due to the differences between the thermal expansion coefficients of starting substrate and epitaxial layer. Thicker layers can also be grown with the aid of partially structured interlayers composed of WSiN, TiN or SiO2 and deposited in an external process, wherein said thicker layers can be subsequently separated as free-standing layers which typically have plastic, concavely bent c lattice planes and surfaces. At or above the interface between starting substrate and epitaxial III-N layer vertical and horizontal micro-cracks can form, which can expand over time and which can lead to breaking of the GaN layer during or after the cooling process.
From investigations by Hearne et al., Applied Physics Letters 74, 356-358 (1999) it is known that during the deposition of GaN on a sapphire substrate an intrinsic tensile stress builds up which increases with the growth. An in situ stress monitoring showed that the tensile stress produced by the growth cannot be measurably relaxed by annealing or thermal cycling. This means inter alia that a stress obtained at the end of the growth of the GaN layer will have the same value again after cooling and reheating to the same (growth) temperature. In Hearne et al. also an explanation of the background, relationships and possibilities for observation of extrinsic (namely generated by different thermal expansion coefficients between sapphire substrate and GaN layer) and intrinsic (namely generated by growth) stress is given.
In order to counter-act the generation of stress, namely of tensile stress, in multilayer structures of III-N layers on foreign substrates with increasing growth of the III-N layer structures in US 2008/0217645 A1 the following measures are undertaken: firstly on a nucleation layer a AlGaN gradient layer is deposited, and secondly relaxed GaAl(In)N interlayers are provided in between nitride layers. Furthermore, when after several epitaxial layers the dislocation density increases excessively in the epitaxial layer structure, in US 2008/0217645 A1 mask layers with for example SiN, MgN and/or BN mask material are used to reduce the dislocation density. Also in further examples and other connections the influence of mask layers on the change of the dislocation density is described, for example in Tanaka et al., Jpn. J. Appl. Phys. Vol. 39, L831-L834 (2000) (in particular with regard to the use of a SiC foreign substrate), in WO2012035135A1 (in particular with regard to the use of a Si foreign substrate) as well as in the publication by Hertkorn et al. (2008) further discussed below.
Napierala et al. in Journal of Crystal Growth 289, 445-449 (2006) describe a process for producing GaN/sapphire templates onto which crack-free thin GaN layers are grown by being able to control the intrinsic stress in the gallium nitride through the setting of the density of gallium nitride crystallites in such a way that stresses in the thin layers can be released by bending. In this process, however, thick layers cannot compensate the pressure during the growth and tend to breaking despite the bending. Richter et al. (E. Richter, U. Zeimer, S. Hagedorn, M. Wagner, F. Brunner, M. Weyers, G. Tränkle, Journal of Crystal Growth 312, [2010] 2537) describe a process for producing GaN crystals via Hydride Vapor Phase Epitaxy (HVPE) in which GaN layers having a thickness of 2.6 mm can be grown in a crack-free manner by setting the partial pressure of gallium chloride, wherein the obtained GaN layers exhibit a multitude of V-pits on the surface. A crystal grown with this process has a thickness of 5.8 mm, it however exhibits longer cracks. Brunner et al. in Journal of Crystal Growth 298, 202-206 (2007) show the influence of the layer thickness on the curvature of the epitaxial III-N layer. The growth of GaN and AlGaN, optionally with InGaN compliance layer, on GaN-sapphire template is investigated. It was found that the concave curvature increases during the growth for GaN and AlGaN with 2.8% and 7.6% of Al mole fraction. Furthermore, the concave curvature increases with rising aluminium content. In addition, the influence of a Si-doped indium gallium nitride layer on the growth of an AlGaN layer with 7.6% of Al mole fraction on a GaN buffer layer is shown. For this purpose on the one hand an AlGaN layer with 7.6% of Al mole fraction is directly grown onto a GaN buffer layer, and on the other hand a Si-doped indium gallium nitride layer as interlayer is grown onto a GaN buffer layer, wherein subsequently an AlGaN layer with 7.6% of Al mole fraction is grown onto the interlayer. It was thus shown that the deposition of a Si-doped indium gallium nitride layer onto a GaN buffer layer leads to compressive stress in the crystal. During this process the initially concave curvature of the GaN buffer layer is transformed into a slightly convex curvature in the course of a temperature reduction, and this convex curvature increases during the further growth by epitaxially growing an In0.06Ga0.94N layer within the same process. During the subsequent deposition of an Al0.076Ga0.924N layer onto this In0.06Ga0.94N layer a concave curvature is eventually obtained, which is comparatively lower than the resulting curvature without In0.06Ga0.94N interlayer.
E. Richter, M. Grunder, B. Schineller, F. Brunner, U. Zeimer, C. Netzel, M. Weyers and G. Tränkle (Phys. Status Solidi C 8, No. 5 (2011) 1450) describe a process for producing GaN crystals via HYPE, wherein a thickness of up to 6.3 mm can be reached. These crystals exhibit slanted sidewalls and V-pits on the surface. Moreover, the crystal lattice has a concave curvature of approximately 5.4 m and a dislocation density, of 6×105 cm−2.
Hertkorn et al. in J. Cryst. Growth 310 (2008), 4867-4870 describe process conditions for forming 2-3 μm thin GaN layers via Metal-Organic Vapor Phase Epitaxy (MOVPE) using SiNx masks deposited in situ. The relationship with a possible influence on defects or respectively the development of the dislocation density is investigated with respect to different locations or respectively positions of the SiNx masks, specifically at 0 (i.e. directly on an AlN nucleation layer) or after the growing of 15, 50, 100, 350 and 1000 nm. As a result it is speculated that a defect termination or respectively a decrease of the dislocation density is most effective when the SiNx is positioned after the growth of 100 nm of GaN. On the other hand, it is being stressed as negative and problematic that the deposition of SiNx directly on or in the vicinity of the AlN nucleation layer produced strongly compressively stressed GaN layers and led to layer deformity—so-called stacking faults, which were visible in the transmission electron microscope and which were further accompanied by a broadening of the D0X line width and the X-ray peaks. Therefore, for avoiding such problems, a second SiNx mask was deposited after 1.5 μm for the screening of the defects. Apart from a reduction of the dislocation density, which however was also described as being associated with disadvantageous effects, the authors did not recognize which parameters being important for the further processing of templates can be influenced in which way by a deposition of SiNx, and especially if and how a later tendency towards crack formation during the growth of further III-N layers and III-N bulk crystals can be suppressed.
DE 102006008929 A1 describes a nitride semiconductor device based on a silicon substrate and the production thereof, including the deposition of an aluminium-containing nitride nucleation layer on the silicon substrate. A process is described which is based specifically on the use of a silicon substrate, wherein it is observed that the growth of semiconductor layers on sapphire substrates is subject to completely different boundary conditions compared to the growth on a silicon substrate. In fact as a result after cooling to room temperature the III-N layer grown according to the system of DE 102006008929 A1 is not compressively, not even nearly compressively stressed, but merely less tensily stressed compared to a conventional III-N layer grown on a silicon substrate.
US 2009/0092815 A1 describes the production of aluminium nitride crystals having a thickness between 1 and 2 mm as well as aluminium nitride layers having a thickness of 5 mm. These layers are described as crack-free and can be used to cut colourless and optically transparent wafers having a usable area of more than 90% for the application in the production of devices and components.
The processes in the above-described prior art have in common that after growth and cooling-down III-N crystals are obtained which are subjected to strong extrinsic and intrinsic stress, whereby cracks or other material defects can develop, which limit the material quality and the processability towards III-N substrates.